Method for lining a threaded nut with a low friction fabric



Dec. 9, 1969 Filed Nov. 8, 1966 IGI W D. CHEESMAN ETAI- LOW FRICTIONFABRIC SABSOSG METHOD FOR LINING A THREADED NUT WITH A 4 Sheets-Sheet lBLOCK DIAGRAM OF METHOD STEPS PROVIDE AN AXIALLY SECTIONED EXTERNALLYTHREADED PREFORM MANDRELFIGIZ.

WRAP THE PREFORM WITH A THIN BARRIER SHEET AND THEN WITH LOW FRICTIONIMPREGNATED FABRIC CONFORM THE FABRIC TO THE CRESTS AND SIDES OF THEPREFORM THREADS BY WRAPPING THE FABRIC ON THE PREFORM WITH A STRING ORCORD.

FIG. 4.

REMOVE STRING OR CORD WHILE POSITIONING WRAPPED PREFORM INSIDE OFNUTFIGG LIGHT PRESS PREFORM SECTIONS TO PLACE FABRIC ON NUT THREADS. FIGS.7A B G C, FIG.8

DRY FABRIC LINED NUT,e.g., IN AN OVEN REMOVE BARRIER SH T cu T FABRIC TOABUT INSIDE NUT.

HEAVY PRESS FABRIC AGAINST NUT THREADS IN STEP-BY-STEP SEQUENCE,

ALSO FIG.|0.

FINAL cuRE RESIN OF FABRICLINING THREADS USING CURING MANDREL ANDHEATING, e.g., IN AN OVEN.

FIG. II. TRIM ENDS OF FABRIC OVERHANGING NUT.

INVENTORS WILLIAM D. CHEESMAN RALPH HI HOEFS J Alma Rm ,Z' a g OR EYS1959 w. 0. CHEESMAN ET AL 3,483,056

METHOD FOR LINING A THREADED NUT WITH A LOW FRICTION FABRIC 4Sheets-Sheet 2 Filed Nov. 8, 1966 INVENTORS WILLIAM D. CHEESMAN RALPH H.HOEFS ATTORNEYS Dec. 9, 1969 w. o. CHEESMAN ET AL 3,483,956

- METHOD FOR DINING A THREADED NUT WITH A LOW FRICTION FABRIC 4Sheets-Sheet 5 Filed Nov. 8, 1966 HQM HZQ

INVENTORS WILLIAM D. CHEESMAN RALPH H. HOEFS Dec. 9, 1969 w. o. CHEESMANET AL METHOD FOR LINING A THREADED NUT WITH A LOW FRICTION FABRIC 4Sheets-Sheet 4.

Filed Nov. 8, 1966 INVENTORS WILLIAM D. CHEESMAN RALPH H. HOEFS UnitedStates Patent US. Cl. 156-189 7 Claims ABSTRACT OF THE DISCLOSURE Amethod of lining a nut with a low friction fabric includes the steps ofwrapping the fabric on a threaded sectional preform, conforming thefabric to the threads of the preform by wrapping with a string or cord,removmg the stirring while placing the preform in the nut, pressing fromthe inside of the preform to press a resin bondable surface of thefabric onto the nut threads, removing the preform sections, furtherpressing the fabric onto the nut threads and curing the resin to finallybond and secure the fabric to the threads of the nut.

This invention relates to a method of making a low friction, precisionfabric lined internally threaded article, i.e. a fabric lined nut.

Load bearing nuts for use for example in heavy duty linear actuatorsmust have low frictional resistance with their mating screw. Lining suchnuts has bee-n contemplated to substantially lessen the frictionalresistance. One known solution is to line the nut with a fabric having alow friction facing material. This, however, is extremely difficult toaccomplish in practice because it is very difficult to conform flexiblelow friction material to internal threads and still obtain the precisionand load bearing capability required. It was not previously thought tobe possible to line the threads of a nut with a fabric and obtainanything like inch tolerance. This invention solves, however, thatproblem in a unique way by lining the nut with a low friction fabric andspecifically, includes the steps of pulling the fabric tight over thecrest and sides of a threaded preform mandrel, inserting the preform andfabric in a nut, securing the fabric to the nut, compacting and curingthe lining so that the final product results in a precision lined nutwhere at least three of the four surfaces of the nut thread (assuming anacme thread) results in a very close tolerance, equivalent to that ofprecision machinedrnuts.

The purpose of this invention, therefore, is to provide a method oflining a nut with low friction fabric material to a precision toleranceof the order of 1 mil, thereby allowing reduced friction with a matingscrew.

To illustrate in greater detail the method of this invention, there isset forth in the following drawings both the method steps as well as thestructure of the apparatus in detail, employed in the method of thisinvention.

In the drawings:

FIGURE 1 is a block diagram of the specific method steps involved inthis invention with descriptive legends and reference to subsequentfigures for the details of the specific steps;

FIG. 2. is a perspective view of a tri-section preform mandrel with boltand washer to hold sections together;

FIG. 3 is a cross-sectional view of the tri-section preform mandrel withvery thin barrier sheet and low friction fabric wrapper thereupon;

FIG. 4 is a perspective view illustrating conforming of the fabric tothe mandrel by wrapping same with a strong string or stout cord;

3,483,056 Patented Dec. 9, 1969 FIG. 5A is an enlarged cross-sectionaldetail view of the low friction fabric wrapped by the cord and made toconform to the thread crests and a portion of the thread sides of thepreform mandrel;

FIG. 5B shows in perspective a partial top view of the wrapped preformshowing overlap of the low friction fabric and a tab of the barriersheet;

FIG. 6 is a perspective view showing the step of removing the string orcord as the preform mandrel is turned and elevated into a nut withouttouching the nut;

FIG. 7A is a perspective view showing the segmented preform mandrelinside the nut with a pressing member pressing down on one segment andwith the nut supported in a cradle;

FIGS. 78 and 7C are perspective views similar to FIG. 7A in showing thepressing member pressing the other mandrel segments;

FIG. 8 is an end elevation view showing the three segment preformmandrel after the fabric has been pressed against the internal threadsof the nut;

FIG. 9 is an end elevation view showing preform mandrel segment B beingremoved from the assembly;

FIG. 10 is an end elevation view showing preform segment A used withpressing member for final pressing of the fabric liner into the threadsof the nut to compact same;

FIG. 11 is a perspective view of lined nut with a curing mandreltherein; and

FIG. 12 is an enlarged sectional view through a section of a linedthread.

The invention will be described in detail below with reference to theaccompanying drawings and with the understanding that the description isof a preferred embodiment.

The nut to be lined in the preferred embodiment may be used as the nutof a linear actuator which includes a complementary screw. The nut isdesigned to have the largest possible area of bearing surface avaliablefor each increment of nut axial length for given major and minordiameters. For example, the nut preferably has an acme thread with fourstarts, and in one successful commercial embodiment the nut threads areof one inch pitch, four and one-half inches minor diameter, five andone-half inches long, and the nut is made of SAE 4340 alloy steel heattreated to 500 BHN. The nut is lined with fabric to fit the screwthreads with zero clearance between their two faces and their majordiameters and some clearance between their minor diameters.

The fabric with which the nut is to be lined internally is of compoundweave with bondable threads predominantly on the back side to contactthe nut and Teflon threads predominantly on the front side to contactthe screw. The Teflon threads are spun or wound of Teflon filaments andmay include some bondable material filaments.

The fabric is first supported on a preform mandrel 14 as in FIGS. 2 and3. The mandrel comprises three segments which are held together by abolt nut and washer assembly 16. The mandrel is formed from a threadedcylinder initially having threads 18 thereon corresponding to thethreads of the nut to be lined with appropriate clearances. Thisthreaded cylinder is cut axially to provide three segments A, B and C(FIG. 3), with a portion of the circumference removed at each axial cutso that the assembled mandrel (which looks like a three-cornered hat) isof a lesser diameter than the internal diameter of the nut and may movefreely into the nut without touching the nut threads. The preformmandrel threads are preferably cadmium plated so that resin bonding thefabric will not stick to them. The fabric to be wrapped on the preformmandrel 14 is a pre-dried, but uncured, resin impregnated fabric 22 witha low friction material such Teflon on one side and Dacron, cotton orthe like, on the other side. An oversized rectangle of this fabric iscut to size for wrapping over the preform mandrel and barrier sheet 20with excess length at both ends. The bias of the filaments of the fabricmay'optionally correspond with the pitch of the threads. After cuttingand while still laid out fiat, the bondable side of the rectangle offabric is roller coated with resin and allowed to air dry to a tackydough-like consitency. This bondable side is to be outside on thepreform so as to contact the threads of the nut. The amount of resin isonly that later needed to mechanically support the fabric threads andhold the fabric in the nut. In other words, the resin is not a filler asthe back of the fabric is to be in direct contact with the nut.

The assembled preform mandrel 14 is first wrapped with a barrier sheet20, such as Teflon 1 mil thick, to further prevent adherence of the lowfriction resin impregnated fabric to the preform. Note that betweenmandrel segments A and C the barrier sheet has an inwardly extending tab20t which will aid in its removal at a later step in the process. Thefabric is then wrapped over sheet 20 around the preform mandrel andoverlaps the start of the fabric to provide the overlap 220, see FIG. B.Note that the portion of barrier sheet 20 opposite tab 20t lies beneaththe overlap 220 and that overlap 220 is located on mandrel segment A andopposite segment B.

The next step shown in FIG. 4 and in detail in FIGS. 5A and B is to atleast partially conform the fabric to the preform and especially conformthe fabric to the crest and a portion of the sides of the threads of thepreform mandrel by wrapping the fabric with a removable strong string orcord 24 positioned in the thread roots. In

pushing the fabric into the thread roots the cord should shorten thelength of the rectangular piece of fabric within predetermined limits.Too much shortening will result in excess ballooning over minor nutdiameter. If, on

the other hand, it is not enough, then the fabric will not bottom in themajor nut diameter. For this purpose the cord or string may be as wideas the thread root requires. Note that there is some excess fabric overthe crests of the mandrel which is not less than the amount needed toallow the fabric to extend across the roots between the threads.

With the preform mandrel disposed vertically and a nut 26 to be linedheld rigidly above the mandrel and in alignment therewth and with theupper ends of the cords 24 tied to cord pullers (not shown) the preformmandrel is next rotated and elevated into the nut while the taut cords24 unwind from the preform mandrel directly under the nut as the preformmandrel is completely inserted within the nut. The purpose of unwindingthe cords just before the mandrel with the fabric thereon moves into thenut is to leave no time for the uncured fabric to droop from the threadson the preform mandrel and so that the fabric is moved into the nutwithout touching the threads of the nut.

Once the fabric is positioned inside the nut with the segments, theentire assembly is laid in cradle 30 with segment B downward. Thesegments are then to be light pressed by a pressure member 28 with apressure of the order of 50 psi. on the mandrel segment to causethematerial to stick to the inside of the nut while supported in cradle 30.The segments are light pressed in a particular sequence as shown andwith reference to overlap 220. As shown in the drawings, segment B ispressed first as shown in FIG. 7A and segment C is similarly lightpressed, to cause a gap to be developedv between segments C and A asshown in FIG. 7B. Finally, segment A is light pressed and this causesthe gap between segments A andC to become larger as shown in FIG. 7C.

Following this initial light pressing operation, all three mandrelsegments A, B and C are in the positions shown in FIG. 8 so that twomandrel segments may then be removed as shown in FlG. 9.l\/ Iandrelsegment A should be removed and either segment B or C should alsoberemoved. The remaining mandrel segment is given an intermediate presswith a press load of 1000 psi. while rotatably stepping the nut in thedirection of the arrow, see FIG. 10, to effect further compacting of thefabric in the nut threads. This stepwise rotation of the nut 26 incradle 30 with pressure'rnember 28 provides non-isostatic pressure onoverlapping steps of about l" each with the remaining segment proceedingtoward and over the tab 201" and fabric overlap 220'and backapproximately to its original positon. 4

I The fabric is now compacted into thenut threads but is not yet cured.The remaining mandrel segment should be removed for and during thenext-step and the Teflon barrier sheet separating the resincontaininglow friction fabric and the mandrel threads is also removed by merelypulling on non-sticking tab 20! and stripping-the sheet.

Next the resin'of the fabric lined'nutis dried by placing the lined nutin an oven. The drying is preferably accomplished at a temperature of280 F. for the'time necessary to dry out the particular resin coatedfabric.;In the case of a typical material this would take in the orderof 15 minutes. This drying step leaves resin with microscopic pores as aresult of the solvents being driven off, such that it is an open-cellcellular material to a limited degree.

The lined nut is then removed from the oven and allowed to cool.Following cooling the fabric overlap 220 is next cut with a knife in astraight axial line over the threads and the edges of the fabric arerepositioned so that they will now abut.

The nut is now replaced in cradle 30 and the fabric is given a heavypress using one preform'mandrel segment in another step-by-step sequenceas illustrated in FIG. 10. The pressure here required is in theorder of2000 p.s.i. to establish a precise thickness to the'layer so that thenut can be screwed onto its mating part. This heavy press is to be surethat all filaments are buried in resin with'only the top sides ofthe'Tefion filaments exposed and to compact all filaments in the openpored resin.

The next step is the final curing. For that purpose and as shown in FIG.11, the lined nut with-excess fabric at the ends of the nut is threadedon a cadmium plated curing mandrel 34 which is externally threaded toduplicate the screw threads of the preform mandrel. The curing mandrelis cylindrical and is proportioned so that it will expand and contractin proper relation to expansion and contraction of the nut. The wholeassembly while standing on the end of the curing mandrel is heated in anoven at a temperature of several hundred degrees Fahrenheit. With atypical material, a temperature. of 325 F. for a period of four hoursmay be required.-

The assembly is now removed from the oven and cooled in air with thecuring mandrel standing on end. Atfer cooling the mandrel is removed byturning the nut off carefully. The selvage edges 22s may now be cutflush with the end of the-nut. The compacted fabric and the cured resinshould have a thickness leaving in the order of only :0005" clearancebetween thenut. and screw for near zero. axial and radial play.Following the trimming of the material or fabric extending out the endsof the nut, the product is finished.

FIG. 12 is a sectionalview through thefinished lined nut, with thethread configuration of-the preform mandrel shown in phantom. It isnoted that the fabric, completely conforms to thethread sides and rootsof vthe nutas it has previously conformed to the crest and sides .of thethreads of the preform mandrel. The preform mandrel had excess clearancein its thread rootproviding excess clearance on the minor. diameter ofthe -nut, .i.e.,-on the threaded crest of the nut.

"As can be seen, this invention. provides a unique method of lining aninternally threaded article such as anut involving the essential step ofplacing a lining fabric on a preform and conforming it over the crestand sides of the preform so that when the preform is inserted into thenut, the fabric is compacted under the nut threads and cured so that thenut will have precision threads therein.

While the production of the instant lined nut has been described wtihdetails as to specific steps, treating times and temperatures, theselimits are only given as illustrations and are not to be deemedlimiting. It will be understood that numerous changes may be made in thedetails of construction, arrangement and operation without departingfrom the scope and spirit of the invention, especially as defined in thefollowing claims.

What is claimed is:

1. A process for lining internal threads of a nut with a low frictionflexible fabric, comprising:

(a) wrapping a preform mandrel having threads thereon corresponding tomating threads of the nut with the low friction fabric which has a resinbondable surface and an antifriction surface, the antifriction surfacebeing positioned adjacent the preform threads,

(b) conforming the fabric to the crest and at least a portion of thesides of the threads on the preform mandrel by wrapping an elongatedflexible material such as a cord around the fabric, the cord positionedin the thread roots of the preform threads,

(c) removing the elongated flexible wrapping material while insertingthe preform with the fabric conformed to the threads thereon into thenut without contacting the threads of the nut with the fabric,

((1) bonding the resin bondable surface of the fabric to the nut bypressing the resin bondable surface against the nut, removing thepreform, and curing the resin of the resin bondable surface to finallybond and secure the fabric to the internal threads of the nut.

2. A process as in claim 1 wherein the pressing ineludes initially lightpressing with a pressure in the order of psi, followed by intermediateheavier pressing with a pressure in the order of 1000 p.s.i., thendrying the resin, and finally utilizing heavy pressing in the order of2000 p.s.i. prior to curing the resin.

3. A process as in claim 2 wherein the preform mandrel is sectional andthe pressing is accomplished using one section of the preform mandrel.

4. A process as in claim 3 further comprising utilizing a barrier sheetbetween the threads of the preform mandrel and the low friction flexiblesheet material to prevent the resin bonding from causing the material tostick to the preform.

5. A process as in claim 2 wherein the curing is accomplished utilizinga curing mandrel positioned adjacent the internal threads of the nut.

6. A process as in claim 1 wherein the preform mandrel threads arecadmium plated.

7. A process as in claim 1 wherein the low friction fabric is in theform of a sheet and the ends of the fabric sheet are wrapped on thepreform in overlapped relationship, and after final bonding and securingthe fabric to the nut the overlapped material is cut off and removed.

References Cited UNITED STATES PATENTS 2,724,672 11/1955 Rubin 156294 XR2,751,237 6/1956 Conley 156-293 XR 2,928,764 3/1960 Magoon -46 XR2,943,967 7/1960 Simon 8546 XR 2,953,418 9/1960 Runton et al. 156-278 XR3,002,770 10/ 1961 Chestnut et al. 285-94 HAROLD ANSI-IER, PrimaryExaminer G. W. Moxon II, Assistant Examiner US. Cl. X.R. 85-46; 156289,293, 312; 28594

